1. Technical Field of the Invention
This invention relates to a method for the purification of methyl tertiary-butyl ether, more commonly referred to as MTBE. More particularly, this invention relates to a method for the separation of methanol from MTBE by distillation in the presence of water.
MTBE is typically produced from a refinery stream containing isobutylene in a C.sub.4 -C.sub.5 diluent and methanol. In the separation of the unreacted methanol in the reactor effluent, the C.sub.4 -C.sub.5 diluent acts as an azeotroping agent to selectively remove the small amount of unreacted methanol into the overhead product of the first distillation column. The bottoms product of this column is fairly pure MTBE contaminated with only small amounts of methanol, t-butanol, and other impurities.
In the one-step MTBE process, t-butanol and methanol are reacted to product MTBE and water. A portion of the t-butanol fed to the reactor system is dehydrated to form isobutylene, which also produces additional amounts of water. The relatively large concentration of water in the reactor effluent does not permit the hydrocarbon-methanol azeotrope to preferentially form as in the typical process described above. Therefore, the bottoms product of the isobutylene removal distillation column contains water, unreacted t-butanol, and essentially all of the methanol. This is a disadvantage in the next column since MTBE forms an unfavorable azeotrope with methanol, making it very difficult to produce an MTBE product relatively free of methanol.
In the present invention, an additional amount of water is added to the second distillation column to break the MTBE-methanol azeotrope and to produce an MTBE overhead product much reduced in the concentration of the methanol contaminant.
2. Prior Art
In U.S. Pat. No. 4,820,877, spearation of methanol from MTBE is accomplished by using a refinery fuel gas to enhance the separation of methanol into the overhead stream of a distillation column.
In U.S. Pat. No. 4,814,517, separation of methanol from MTBE is accomplished by using a silica gel to preferentially adsorb methanol from an MTBE stream and by periodically regenerating the silica gel.
In U.S. Pat. No. 4,798,674, separation of methanol from MTBE is accomplished by using a membrane of crosslinked polyvinyl alcohol or a quaternary ammonium ion resin. Methanol preferentially permeates through the membrane increasing the MTBE concentration of the charge liquid.
In U.S. Pat. No. 4,759,850, separation of methanol from MTBE is accomplished by reverse osmosis.
In U.S. Pat. No. 4,440,963, separation of methanol from MTBE is accomplished by adding an agent such as 2-methyl pentane or Freon 113 to form an azeotrope with methanol. This azeotrope is recovered overhead giving a methanol-free MTBE bottoms product.